In drilling and servicing wells, various well fluids are usually employed, for example, drilling mud. The drilling mud is pumped down a string of tubing to which is attached a drill bit. The drill bit is provided with openings above the cutting edge through which the mud passes outwardly and returns to the surface by way of the annulus formed by the string of drill tubing and the well bore wall or casing where the well has been provided with casing. The drilling mud serves a number of functions including: carrying the cuttings of the drill bit to the surface; plastering the well bore to prevent sloughing of earthen materials from the bore; and, providing a weighting fluid in the well bore.
Upon completion of the drilling, the drilling mud is removed from the well by circulation or swabbing. Typically it is replaced by completion or packing fluid. The replacement of the drilling mud by the completion or packing fluid is usually accomplished by first injecting a preflush into the well. The preflush forces the drilling mud out and at the same time cleans the well of mud residue. The preflush is then followed by an injection of a completion or packing fluid which in many instances comprises a high density aqueous brine. Typically these brines contain high concentrations of sodium bromide, sodium chloride, calcium bromide, calcium chloride, zinc bromide, zinc chloride or mixtures thereof. These aqueous brines typically have densities in the range from about 8 to about 19 pounds per gallon.
The aqueous brine should be free of solid contaminants. Solids in a completion fluid can cause serious injury to a producing formation by plugging the pore spaces therein or even by plugging the perforations and channels provided to induce fluid flows between the formation and the well bore. If there are solids in a packer fluid, in time the solids can precipitate and settle upon the packer. Such solids deposits make it difficult to disconnect the tubing from the packer resulting in a costly well workover.
Although it is desirable to have an aqueous brine that is free of solid contaminants, it is difficult to keep the brine contaminant-free. Solid contaminants can be introduced into the brine prior to its injection into the well and during the actual injection process. Although it is possible to manufacture the brine at the well site, it is difficult to do so because of the extremely high concentrations of salt. Therefore, the brine is typically manufactured and transported to the well site. This means that solid contaminants can enter the aqueous brine when it is being stored at the manufacturing site, when it is transferred to the vehicle (e.g., the truck or boat) in which it is transported to the well site, and when it is transferred into the well site storage facility prior to injection into the well.
The brine can also become contaminated when it is injected into the well. The pumping equipment used to inject the brine can have solid contaminants remaining therein in spite of a preflush step. Also, the drilling mud tends to form a filter cake on the wall of the well bore which will then contaminate injected brine. The presence of this filter cake is undesirable when a cement slurry is injected into the annulus between the well tubing and the well bore wall. The purpose of the cement slurry is to bond the tubing to the walls of the well bore. This purpose is frustrated if a filter cake is present because the cement may not adhere to the walls of the well bore because this filter cake provides a poor adhesive surface. Therefore, it is desirable to remove as much of the filter cake and other solid contaminants from the well system as possible in the preflush step.
The prior art has dealt with contaminants in fluids in a number of ways. For example, U.S. Pat. No. 4,451,377 issued May 29, 1984 to S. R. Luxemburg, U.S. Pat. No. 4,218,316 issued Aug. 19, 1980, to J. L. Watson et al., U.S. Pat. No. 4,127,482 issued Nov. 28, 1978, to J. L. Watson et al., and U.S. Pat. No. 3,737,037 issued June 5, 1973, to L. Bone III, all disclose the use of polymeric flocculating agents to clean oil contaminated fluids or to flocculate solids in contaminated water. In general these patents disclose a process whereby a polymeric flocculant is added to a contaminated fluid. Flocs then form from the contaminants and are removed.
In order for the flocculating agents to function, there must be a charge associated with the contaminant particles. However, in a highly concentrated brine, charge-charge interactions cannot be used to effect the contaminant removal because the available particulate surface charge is essentially zero. This is because the electrical double layer is extremely compressed due to the high ionic strength of the brine solution. Furthermore, flocculation by known flocculating agents, such as polyacrylamide and polyethylene oxide, is difficult because these polymers are not readily soluble in concentrated brine.
A successful procedure for removing solid contaminants from aqueous brine uses an agglomeration agent. In U.S. Pat. No. 4,515,699 issued May 7, 1985, to Oliver, Jr. et al. and U.S. Pat. No. 4,456,537 issued June 26, 1984 to Oliver, Jr., et al. a process is disclosed for removing solid contaminants from brine using an aliphatic alcohol and a surfactant. The surfactant is selected from the group consisting of amines, amides and aliphatic amine oxides. The preferred surfactant is reported to be the amide reaction product of diethanolamine and an organic fatty monobasic acid of the general formula C.sub.n H.sub.2n+1 COOH, where n is 12 to 18.
An advantage of the type of agglomerating agents disclosed in U.S. Pat. Nos. 4,515,699 and 4,456,537 is that they are not only useful for removing solid contaminants from aqueous brine, but also can successfully function as a preflush. In U.S. Pat. No. 4,453,598 issued June 12, 1984, to Singer et al. and U.S. Pat. No. 4,474,240 issued Oct. 2, 1984, to Oliver, Jr. et al., it is taught that the same agglomerating agents disclosed in U.S. Pat. Nos. 4,515,699 and 4,456,537 can also successfully function as preflush agents. That is, they can be diluted and used to flush out drilling muds prior to injecting solids-free aqueous brines or other well fluids.
However, the surfactants disclosed in these patents suffer from a number of disadvantages. First, because the starting materials are preferably pure acids, the manufacturing costs of the compositions are high. Second, when prepared in the way disclosed in U.S. Pat. Nos. 4,515,699, 4,456,537, 4,453,598 and 4,474,240 these surfactants contain not only amides but also acid/amine salts. It has now been found that this can lead to poor agglomeration. In fact, in some cases the compositions actually disperse solids rather than agglomerating them. And third, it is preferable to have a surfactant in which the concentration of amides is higher than that attained in the surfactants disclosed in these patents.
In summary, the prior art agglomerating agents are expensive to produce and their effectiveness can be unsatisfactory. In addition, in some cases they have been found to disperse, rather than agglomerate, solid contaminants. Therefore, it would be advantageous to have agglomerating agents that are relatively simple and inexpensive to produce, and yet perform as well or better than prior art agents. It has now been found that these disadvantages can be minimized by the instant invention.